A thin-film transistor has conventionally been used in a flat panel display device such as a liquid crystal display device or an organic EL display device. In recent years, a flat panel display device has been required to respond to increasing demands for a larger screen, a higher resolution, and a higher frame rate, etc. Some flat panel display devices use an oxide semiconductor layer that can achieve high mobility as a semiconductor layer of a thin-film transistor. In particular, an oxide semiconductor layer made of IGZO, for example, has gained attention as an oxide semiconductor layer satisfying the aforementioned demands as it can be formed in a large area under a relatively low temperature and consumes a small current.
To reduce the resistance of a source region and that of a drain region of the aforementioned oxide semiconductor layer, this oxide semiconductor layer is formed by a process using hydrogen plasma, for example, in which a gate electrode (gate metal) of a top gate type is used as a mask. However, this plasma process makes it difficult to control the diffusion length of hydrogen. This disadvantageously also reduces the resistance of a channel region, making it difficult to form a thin-film transistor of a short length.